Certain drive systems are subject to torsional stresses which are stored as reactive torque in a drive train. When drive power to the system is interrupted, the reactive torque is released as back-spin and, if an uncontrolled release of torque occurs, personal injury and/or property damage can result. For example, deep well submersible pumps such as progressing cavity pumps driven by sucker rod strings are commonly used to pump oil from deep wells. The drive string for these submersible pumps usually have a relatively small diameter of 3/4 to 1 1/8 inches. Such drive strings are commonly used in wells that vary from 1,500' to 6,000' in depth, 3,000' being a common average. Progressing cavity pumps include a stator which is attached to a production casing at the bottom of a well and a rotor which is attached to a bottom end of the drive string. Progressing cavity pumps are frequently used to pump viscous crude oil which is often laden with sand or other impurities. As a result, the elongated drive string is subject to considerable torsional force. This torsional force is stored in the elongated drive string as reactive torque. In a 3,000 foot string, as many as several hundreds of revolutions of torsion can be stored in the string if viscous sand laden crude oil is being pumped. When power is interrupted to the drive string, the reactive torque is released. Unless the release of reactive torque is controlled, costly and undesirable damage to equipment and/or personal injury to workmen in the vicinity can result. This is particularly true if an electric motor is used as a power source because such motors offer almost no resistance to reverse rotation.
If drive power is interrupted to the drive string of a progressing cavity pump, the reactive torque is desirably released in a controlled fashion. Brakes which simply prevent the release of the reactive torque in the drive string are unsatisfactory for two reasons. First, it is preferable that in the case of an electric motor drive, the motor restart unattended when power is restored. In order to ensure a successful unattended restart, the motor must start without load. If the reactive torque in the drive string is not released prior to restart, the motor may not be capable of restarting and the motor may be damaged as a result. Second, if pump repair or replacement is required any unreleased torque in the drive string can be extremely dangerous for unaware workmen. Severe personal injury can result from the unintentional release of reactive torque in such drive strifes.
Consequently, braking systems have been invented to controllably release the back-spin due to reactive torque in elongated drive strings. The most common back-spin control equipment now in use is a fluid brake that includes a pump engaged only when reactive torque is released from the drive string. The pump is used to circulate hydraulic fluid or lubricating oil from a reservoir to a bearing case through a restricted orifice. The resistance of the fluid created by the restriction serves to control the release of reactive torque. Nonetheless, this apparatus is subject to several disadvantages which include:
1) Practically all of the stored energy dissipated by the brake is converted into heat in the circulated fluid. The heat tends to break down the fluid, resulting in a loss of lubricating quality which may cause mechanical failure of the pump. The excess heat can also damage seals and the loss of lubricating quality can eventually damage drive bearings and gears used in the assembly. PA1 2) The pumps are typically mounted internally in an oil bath and are therefore difficult to access for repairs and maintenance. PA1 3) Most such brakes are not adjustable to accommodate various load levels so that the same brake capacity is provided regardless of the length of the drive string or the reactive torque stored in the drive string. PA1 4) Generally, pumps of adequate capacity are too large and expensive to be used for this application. As a result, small pumps that are inadequate to withstand the torques encountered are relied upon, often with unsatisfactory results. PA1 a brake disc mounted on the shaft for rotation with the shaft; PA1 a fluid actuated brake mechanism adapted to engage the brake disc and retard rotation of the disc and consequently to retard rotation of the shaft; PA1 a bi-directional pump for pumping fluid from a reservoir, the pump being driven by the shaft; PA1 a control manifold for directing the fluid from the pump back to the reservoir when the shaft turns in a first direction and for directing fluid to the brake mechanism to retard rotation of the shaft when the shaft turns in the opposite direction, PA1 a shaft coupled with the elastic member to be axially rotatable with the elastic member in either direction; PA1 a brake disc mounted on the shaft for rotation with the shaft; PA1 a fluid actuated brake mechanism adapted to engage the brake disc and retard rotation of the brake disc and consequently to retard rotation of the shaft and the elastic member; PA1 a bi-directional pump for pumping fluid from a reservoir, the pump being driven by the shaft; PA1 a control manifold for directing the fluid from the pump back to the reservoir when the elastic member turns in a first direction to induce torsion in the member, and for directing fluid to the brake mechanism when the elastic member stops turning in the first direction and stored reactive torque is released from the elastic member under torsion, PA1 a shaft coupled with the drive string and rotatable therewith; PA1 a reservoir surrounding and rotatably supporting the shaft; PA1 a coupling for connecting the reservoir to a wellhead assembly of the well; PA1 a brake disc affixed to the shaft and rotatable therewith; PA1 a fluid actuated disc caliper that straddles the brake disc for engaging the brake disc and retarding rotation thereof, and consequently for retarding rotation of the shaft and the drive string; PA1 a bi-directional pump for pumping fluid from the reservoir, the pump being driven by a gear attached to the shaft; PA1 a control manifold for directing the fluid from the pump to the reservoir when the drive string is driven in a direction to drive the down hole pump, and for directing fluid to the brake caliper when the drive string is no longer driven and reactive torque is released from the drive string to turn the shaft in an opposite direction.
Other brake systems for controlling reactive torques have been invented. Those brake systems are based on centrifugal braking principles. Examples of such braking systems are found in U.S. Pat. Nos. 4,216,848 which issued to Toyohisa Shiomdaira on Aug. 12, 1980; 4,797,075 which issued to Wallace L. Edwards et al on Jan. 10, 1989 and 4,993,276 which issued to Wallace L. Edwards on Feb. 19, 1991. The brakes disclosed in these patents all include brake shoes which are mounted within a housing and are therefore difficult to access and maintain. Such brakes require frequent maintenance when they are used to stop the reverse rotation of drive strings in a controlled manner. In addition, they are mechanically complicated and include custom-made moving parts which are costly to manufacture and expensive to keep in inventory.